The present invention relates to the manufacture of pipes and in particular, but not exclusively, to the manufacture of extruded pipes having an inner layer of a corrosion and/or erosion resistant material.
It is known to manufacture pipes by forming a tubular metal billet of volume equal to that of the eventual pipe and extruding the billet through an annular die of appropriate dimensions. This technique can also be used to manufacture pipes having a layer of cladding on their internal surface. Such pipes are widely used to provide a cost-effective solution for corrosion and/or erosion protection for internal surfaces of pipes. The base material of the pipe can be produced from relatively inexpensive material (typically carbon steel) and the liner or cladding can be chosen to take into account the service condition and may, for example, be corrosion or erosion resistant. The material forming the cladding layer is usually much more expensive that the base layer but by providing only a layer of cladding material the cost is significantly reduced.
Such clad pipes are currently produced by inserting a tube of corrosion resistant cladding material into a tubular billet (typically of carbon steel) and extruding the composite billet thus formed in the conventional manner. The resultant pipe is formed with an internal layer of cladding and the changes which the billet undergoes during extrusion cause the cladding material and the base material to be metallurgically bonded to one another. Although the known technique is very useful it is known to produce inconsistencies in bonding and because of this the eventual wastage from the completed product is high. Furthermore, it has been found that by using this method it is very difficult to control the clad thickness, since the cladding material migrates into the base material by a disproportionate amount, resulting in thinning of the base material and thus losing the design capabilities.
It is an object of the present invention to provide a process for producing clad pipe which overcomes or alleviates the problems with the prior art.
In accordance with the present invention a process for producing clad pipe comprises forming a hollow body of base material, bonding a cladding material metallurgically to the base material to form a composite body and subsequently extruding the composite body to form a pipe.
Preferably, the cladding layer is metallurgically bonded to the base material by depositing a weld overlay on the base material. A suitable method of depositing such a weld overlay would be to use the PROCLAD(trademark) technique of Forth Tool and Valve Limited of Glenrothes, Fife, Scotland.
By metallurgically bonding a cladding layer to the base material it is possible to validate the integrity of the bond prior to extrusion. It has also been found that the extruded product thus formed has fewer bonding flaws compared with the pipes produced in the prior art method. Moreover the finished product exhibits low dilution characteristics in the clad layer, which provides optimum properties in the clad layer to combat against erosion, corrosion or the like.
Preferably, the cladding layer, e.g. the weld overlay, is machined prior to extrusion taking place. The integrity of the metallurgical bond between the cladding layer and the base material may also be validated (e.g. by x-ray or ultrasonic inspection) prior to extrusion.